The molecular basis of the switching of isozymic species during myogenesis or muscle cell differentiation will be investigated in this project. Creatine kinase is a dimeric enzyme which generates ATP, which is essential for muscle contraction. It has been widely used as a marker for myogenic cell differentiation and for studying isozyme switching. The isozymic forms are Mm (of skeletal muscle), BB (of chicken heart, brain and gizzard) and MB (in small amounts in heart) where M and B are the polypeptide products of different genes. Since the two main isozymes MM and BB predominate in different tissues, their syntheses are regulated differently. In order to investigate the molecular basis of the switching of the two forms during differentiation, DNA complementary to the M and B mRNAs have been cloned. The isolation of larger cDNAs encoding the mRNAs and of corresponding genomic sequences is proposed. A comparison will be made of the structure of the two genes, in order to ascertain why they are so differently regulated. The physiological factors which are involved in their differential regulation will be identified. Human DNA will be tested for homology to the large chick cDNA sequences in case restriction polymorphisms can be identified. The quail myoblast transformation system of Fiszman will be used to introduce creatine kinase sequences into cultured cells with the long term view of studying what factors may be involved in induction of CK during myogenesis. We expect that these experiments will help to show why the two enzymes with the same activities are so differently synthesized in different tissues.